Papermaking machines generally may be used to manufacture paper products by suspending cellulosic fibers of appropriate length in an aqueous medium, forming the fibers into a wet web, and then removing most of the water from the fibrous web. In the manufacture of absorbent paper products, such as bath tissue, facial tissue, paper towels, paper napkins, and wipers, creping may be performed in order to impart desired aesthetic and performance properties to the resulting paper product. Creping is a process of mechanically foreshortening a fibrous structure in the machine direction in order to enhance bulk, stretch, and perceived softness of the resulting paper product.
After forming the wet fibrous web, partial drying of the web may be achieved by various known methods, such as conventional wet pressing (CWP) or through-air-drying (TAD). The semi-dry fibrous web then may be transferred via a pressure roll to a large, rotating cylindrical dryer, known in the industry as a Yankee dryer, for further drying followed by creping. In particular, the fibrous web may be adhered to the surface of the heated Yankee dryer and rotated therewith to facilitate water removal by evaporation, which may be aided by a dryer hood positioned about the Yankee dryer. The substantially dry fibrous web may be removed from the Yankee dryer surface by a creping blade, a type of doctor blade configured to crepe the fibrous web as the web is separated from the Yankee dryer. Finally, the creped web may be directed through calender rollers and then wound onto a reel prior to further converting operations, such as embossing.
A coating generally may be applied to the cylindrical surface of the Yankee dryer to facilitate adhesion of the fibrous web thereto for adequate drying and creping and also to protect the Yankee dryer surface from corrosion and direct contact with the creping blade. The coating may be made up of various materials, including one or more adhesives, releases, and/or modifiers, as may be desired in certain applications. Excessive build-up or uneven distribution of the coating, particularly creping adhesives, may be addressed by a cleaning blade, a type of doctor blade positioned after the creping blade and configured to remove a portion of the coating from the Yankee dryer surface after removal of the dry fibrous web. Additional coating materials may be applied to the Yankee dryer surface via a spray boom positioned between the cleaning blade and the pressure roll in order to maintain an adequate layer of the coating for subsequent adhesion of the semi-dry fibrous web and creping.
The creping blade generally experiences wear during continued operation of the papermaking machine, which may compromise the creping process and affect the quality of the resulting paper product. Accordingly, periodic replacement of the creping blade may be required to maintain desired creping performance and characteristics of the paper product. During a change-out of the creping blade, the fibrous web adhered to the Yankee dryer may be engaged by a cut-off blade, a type of doctor blade positioned before the creping blade and configured to cut the fibrous web and direct the web to a pulper until the change-out is complete and creping is resumed. The frequency of creping blade change-outs may vary depending upon the operating conditions, the type of blade being used, and the type of paper product being produced.
Various mechanisms have been developed for positioning and loading doctor blades, such as creping blades, cleaning blades, and cut-off blades, against the coated surface of a Yankee dryer. For example, certain existing papermaking machines may include one or more loading cylinders for each doctor, which may operate via air pressure to force the respective doctor blade into engagement with the coated Yankee dryer surface to perform its respective function. The loading pressure of the cylinders on the respective doctor may be visually observed via simple dial pressure gauges and manually adjusted via pressure regulators in an effort to improve performance of the doctor blade. However, such doctor blade loading systems may present several shortcomings.
First, according to existing doctor blade loading systems, the dial pressure gauges may not provide accurate readings of the actual loading pressures of the cylinders, and the pressure regulators may not allow for precise adjustment and control of the loading pressures. Second, such doctor blade loading systems may lack any means for tracking changes in the loading pressures of the cylinders and for indicating whether such changes were the result of operator adjustment at specific times or merely drift of the loading pressures over time. Accordingly, troubleshooting of certain performance problems, such as excessive coating build-up or uneven coating distribution on the Yankee dryer surface, excessive wear of the creping blade, or inadequate creping of the fibrous web, may be difficult to address. Third, existing doctor blade loading systems may lack any means for monitoring vibration of the doctor blades and making necessary adjustments to address blade “chatter,” which may cause inadequate creping of the fibrous web as well as damage to the coating layer, the Yankee dryer surface, and/or the doctor blades. These shortcomings may result in low quality paper product and increased wear of the doctor blades, necessitating frequent troubleshooting of the creping process and change-outs of the blades, which ultimately increase operating costs and decrease operating efficiency.
There is thus a desire for improved doctor control systems for papermaking machines and related methods for controlling loading of doctor blades and monitoring vibration of doctor blades. Such control systems and methods should address one or more of the shortcomings described above in order to decrease operating costs and increase operating efficiency of the papermaking machine.